The increasing complexity of devices on a workpiece forces continuous improvement in workpiece processing apparatus. For example, parameters that were adequate for previous geometries may no longer be acceptable. The process uniformity parameters for these shrinking devices impose more stringent constraints on controlling how species arrive to the workpiece. For example, even a few degrees in ion angle skew may lead to unacceptable consequences on device performance.
This problem arises in cases where the plasma sheath above the workpiece is not flat. This can occur as a result of voltage mismatch between the workpiece surface and the shield ring, a geometrical mismatch between these components, or both types of mismatch. Geometry and voltage may also drift during tool operation. For example, the geometry of the shield ring may drift due to etching effects during tool operation and voltage may drift due to charging effects.
As an example, in some workpiece processing apparatus, there is a relationship between the rate and angle at which a portion of the workpiece is etched and the distance from the center of that workpiece. This may be referred to as radial uniformity. Stated differently, the shape of the plasma sheath above the workpiece may change as a function of the distance from the center of the workpiece. This change in shape may be most pronounced at the outer edge of the workpiece, and may affect the angle at which ions strike the workpiece along that outer edge.
Various techniques have been attempted to address this issue of radial uniformity. However, radial non-uniformity still exists and is becoming a more significant issue as geometries continue to shrink.
Thus, it would be beneficial if there was a method and an apparatus which could correct or compensate for this radial non-uniformity along the outer edge of the workpiece. More particularly, it would be beneficial if there were a method and apparatus for adjustable control of the plasma sheath shape so as to adjust ion angles.